Aluminium strips must fulfil a plurality of requirements simultaneously in order to provide an adequate quality for lithographic printing plate supports. One of the most important properties of the aluminium strip, which must be fulfilled, is homogenous behaviour in an electrochemical roughening. A superficial roughening of the aluminium strip must lead to an unstructured appearance of the aluminium strip with no streakiness effects. A photosensitive layer is applied to the roughened structure, which depending on the type of application, is burned in after application at a temperature of 220° C. to 300° C. for between 3 and 10 minutes. Typical combinations of burn-in times are for example 240° C. for 10 minutes, 260° C. for 6 minutes, 270° C. for 7 minutes, and 280° C. for 4 minutes. The strength loss of the printing plate supports after burning in must be minimal, so that they can still be handled well and clamped easily in the printing apparatus. In the case of large format printing plate supports in particular, handling after burning in the photosensitive layer causes a problem. Finally, the printing plate must later, during use, survive as many printing cycles as possible so that the aluminium strip must have as high a flexural fatigue strength as possible. As well as these general requirements for the use of a printing plate support, for example European patent application EP 2 192 202 A1 investigates how an aluminium alloy strip can be set to a desired final strength, so that for example a coil set present in the aluminium strip can be eliminated again and at the same time high alternating bending cycles and good roughening properties can be provided. The object could be achieved here by the selection of the intermediate annealing thickness depending on the aluminium alloy composition.
DE 699 20 831 T2 describes a method for producing strips for lithographic printing plate supports in which a magnesium-free aluminium alloy is processed using cold rolling passes with pass reductions above 50%. Magnesium contents above 0.02% by weight are considered problematical in relation to recovery of the cold-rolled strip and the occurrence of excessively high strengths after cold rolling.
JP H11229101 also discloses the processing of magnesium-free aluminium alloys, which contain magnesium solely as a contaminant with levels of maximum 0.05% by weight. Higher magnesium contents are considered problematical.
In the production of aluminium strips for lithographic printing plate supports, today the main focus lies on aluminium alloys which contain magnesium. It has been found that magnesium offers advantages in particular in relation to fatigue strength when using the printing plate supports and the roughening of the printing plates. Therefore, magnesium is added to the aluminium alloy up to a precisely defined level.
A further focus of development is the production costs for the printing plate supports. By minimising the layer thickness of the photosensitive layer and the thicknesses of the support materials for the printing plates, i.e. the thickness of the aluminium strip for lithographic printing plate supports, to less than 0.3 mm, optimisation has already been achieved in relation to production costs in manufacture. In production of lithographic sheets, cold rolling is considered critical since it is the final process which determines the surface topography of the lithographic sheet. For cold rolling, working rolls achieving a so-called “mill finish” surface, i.e. polished working rolls, are used. Because of the very high requirements for the later surface quality, cold rolling frequently takes place on roll stands with a single cold rolling pass using the following steps:                uncoiling of the aluminium strip from a coil with an uncoiling reel,        rolling of the aluminium strip using a roll stand with a single cold rolling pass, and        coiling of the cold-rolled aluminium strip.        
Because of the temperature development in cold rolling due to the forming energy applied, strips for lithographic printing plate supports are not usually rolled in roll stands with multiple passes. Maximum control of the individual cold rolling passes is desired. With a single cold rolling pass, it is sometimes however necessary to cool the strips in the coil after each cold rolling pass until they can be subjected to the next cold rolling pass. If the pass reduction in a cold rolling pass is too high, material can break away from the surface of the aluminium strip in regions, which leads to surface defects or a streaky appearance of the surface. Because of the risk of surface defects, the specialist sector has previously turned away from using high pass reductions above approximately 50% pass reduction per cold rolling pass in the case of magnesium-containing aluminium alloys. As a result, in typical production of lithographic printing plate supports with final thicknesses in the range 0.2 mm to 0.4 mm, previously at least four cold rolling passes were required.
On this basis, the object of the present invention is to provide a method for producing an aluminium strip for lithographic printing plate supports comprising magnesium-containing aluminium alloys, with which aluminium strips for lithographic printing plate supports can be produced with high quality and costs can be reduced at the same time.